Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. The appended claims are not intended to be limited, however, to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing or for sensing electrical signals produced by the heart or for both pacing and sensing in which case a single lead serves as a bidirectional pulse transmission link between the pacemaker and the heart. An endocardial type lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled or wound conductor surrounded by an insulating tube or sheath couples the connector pin at the proximal end with the electrode at the distal end.
When terminating a wound conductor to an associated electrical element such as a proximal end connector pin, a heart tissue stimulating electrode at the distal end of the lead, a blood oxygen sensor, or other such elements within the lead assembly, there is often no way to statistically ascertain the structural integrity of the termination. These joints must have a high degree of reliability for the implantable product to be acceptable for long term implants such as endocardial type pacing leads. In the past, the only way to verify the joint was to immobilize the mating part and pull on the wound conductor and this technique has been used as the chief test method. The major problem with this approach is that as the winding is pulled unequal tension is applied to the individual strains of the wound conductor. As increased tension is applied to the coil, often one strain breaks sooner than the others yielding erratic test results. The present invention provides an approach that overcomes this test method problem while at the same time providing a very reliable and secure connection between a wound element and a mating component.
Another problem associated with connections between wound elements and mating components in present day lead assemblies arises from the use of different alloys for the wound elements and mating components. Since dissimilar alloys have different melt temperatures such connections are difficult to weld. Moreover, as lead sizes decrease, problems of manufacturability arise. This is particularly true where crimping is employed to secure the wound component to a mating element. See, for example, U.S. Pat. No. 4,953,564 which discloses a cardiac pacing lead having an extendable fixation helix electrode that is mechanically and electrically connected to a rotatable conductor coil by squeezing the helix and coil together between a crimping sleeve and a crimping core. As the sizes of body implantable leads and their constituent parts become smaller, crimping becomes more difficult because the crimping tools cannot be made sufficiently small. Moreover, the same number of lead windings are not always subjected to the crimping action so that failure stress differs from lead to lead.